Disclosed in JP2004-230946A is an example of a motion control device for a vehicle. In the motion control device for the vehicle disclosed in JP2004-230946A, an allowable value of a lateral acceleration (i.e. allowable lateral acceleration), which acts on the vehicle when the vehicle travels through a curve, is determined on the basis of a speed of the vehicle (i.e. a vehicle speed) and a road surface friction coefficient. An appropriate vehicle speed for the vehicle traveling through the curve is determined on the basis of the calculated allowable lateral acceleration and a radius of curvature of the curve. Then, the motion control device for the vehicle disclosed in JP2004-230946A executes a speed reduction control for reducing the vehicle speed on the basis of the vehicle speed and a position of the vehicle relative to the curve, so that the vehicle travels through the curve at the appropriate vehicle speed.
In a case where the road has an increasing gradient on the curve, the vehicle speed is expected to be reduced due to an action of gravity. Hence, a sense of security a driver feels (which is hereunder referred also to as a sense of security of a driver) may be high when the vehicle advances to the curve. On the other hand, in a case where the road has a decreasing gradient on the curve, the vehicle speed is expected to be increased due to the action of gravity. Hence, the driver may feel less secure (i.e. the sense of security of the driver is low).
Further, in a case where visibility of the curve is poor because relief of the curve is great, because an elevation of an inner side of the curve is higher than the elevation of the curve, or because of an existence of a tall building at the inner side of the curve, the sense of the security of the driver may be low. Furthermore, the level of the sense of security felt by the driver may fluctuate depending on the driving conditions even if the vehicle travels through the curve while receiving a predetermined same level of lateral acceleration. For example, the sense of security of the driver may be high in a case where the vehicle travels the curve having a small curvature radius of the curve at a low speed. On the other hand, the driver may feel less secure when the vehicle travels the curve having a great curvature radius of the curve at a high speed even if the vehicle travels through the curve while receiving the predetermined same level of the lateral acceleration. Accordingly, the sense of security of the driver fluctuates depending on the gradient of the road on the curve, visibility beyond the curve, a bending grade of the curve (the vehicle speed when traveling through the curve) and the like.
It is considered to be preferable that the appropriate vehicle speed (or, the allowable lateral acceleration), which is set in the above-mentioned speed reduction control, is determined in view of the sense of security of the driver. In other words, the appropriate vehicle speed (or, the allowable lateral speed) may be preferably set in a manner where the greater the sense of security of the driver is, the greater value the appropriate vehicle speed (or, the allowable lateral acceleration) is set to be. On the other hand, in the motion control device disclosed in JP2004-230946A, the appropriate vehicle speed (or, the allowable lateral acceleration) is determined without consideration for the sense of security of the driver.
A need thus exists to provide a motion control device for a vehicle which is not susceptible to the drawback mentioned above. More specifically, the purpose of the present invention is to provide the motion control device (a speed reduction control device) for the vehicle for executing a speed reduction control in view of a sense of security a driver feels when the vehicle advances to a curve.